The real problem behind is that, after [6958] iframe dialog use both the attribute and inline style to present dimension, which are conflicting with each other, but why haven't it been aligned with other style-based dialog (like image and table)? Well that's just because a historical reason of fake object which are more convenient to manipulate attributes than with styles.

The proposed patch eliminates attribute based dimension and introduces necessary changes to make this happen easier.

Oops, back-compact is broken by my previous patch, but I'm afraid that force convertion wouldn't be an option, as it mangles user's content on data output, let's temporarily follow the current conversion in image dialog - attributes are dropped only if the dimension is modified by dialogs. I'll come with a new patch.